Heat load calc help

I'm about to pull the trigger on a new boiler/indirect water heater. I've narrowed it down to the Lockinvar Knight, just need to decide between the 80000 and 110000. It's a 2500 sq ft house with half in a daylight basement, in Anchorage, AK. It's very air tight (full envelope spray foamed), but the r value is weak, with 3" sprayed on the underside of the roof sheathing of the vaulted ceiling hot roof (~r18), and maybe only a couple inches on the walls and I'm guessing nothing underneath the floor downstairs. A lot of glass on the north side (where the view is) with old 1978 dbl pane glass. There is also a large 4 car garage which is drafty but will be tightened up. There is 70' of standard (I believe 600btu/ft) tube and fin baseboard on each floor with another 25' in a hot tub room. The hot tub is also heated with the boiler, but most of the loss would be in the heated space so I don't think it adds much for load. The garage will be heated with a 61000 btu radiator/fan unit, kept at approx 45 degrees most of the time. Design day outdoor temp for Anchorage is -18f, I believe.

There is a govt program up here that reimburses you for energy improvements. They use AK warm software for their heat load calc, which uses measurements of windows, floor space, solar gain, etc. as well as a blower door test and it came up with some numbers, as follows:

"The above value is the required amount of heat delivered to the conditioned spaces. If you need to determine the required input rating of the heating system, you must increase this figure to account for the inefficiency of the heating distribution system, the inefficiency of the furnace or boiler, and an appropriate sizing safety margin."

I don't fully understand all of the above. I got the idea that they add a fudge factor or two. And this is prior to doing any of the improvements they recommend, most of which I will do. At the end of their list of improvements, the design heat loss, Btu/hr is listed at 79563. I don't know how accurately the program can predict all this. I know when we closed the garage off, the door fan really bogged down, and he said the living space was REALLY tight. No infiltration could be felt on electrical boxes, etc. unlike my last house.

I want to double check with my actual gas usage, but am unsure of the math there either. Last year (from the gas bill) my highest usage month was Jan at nearly 400CCF, with avg temp of 12f. In the summer with no heating (except the stupid boiler keeps itself hot all summer) it runs about 110 ccf (for the clothes dryer, cook stove, hot tub and 75 gal gas water heater). Leaving I guess approx 300CCF for heating in Jan. The current boiler is a giant Utica with 299999 input. The output spot on the tag is blank, but it also has spaces for DOE heating cap: 225000 and water: 195700Btu, so I'm not sure how to use these numbers to figure my gas usage and heat load? Or how inefficient that grossly over sized thing is? There is also a gas fireplace insert downstairs that could help out on those coldest days, and another fireplace upstairs that we plan to install a wood insert in.

I would really like to properly size my system. I am leaning toward the 80000 Btu unit, but everyone recommends the 11000 (to be safe!) With my current baseboard at 600Btu/ft there is about 99000Btu capacity, plus the garage heater at 60000BTU, so there is enough radiant capacity for either boiler. In the future I want to add radiant flooring on the upper level (with warmboard or something similiar). And we are replacing windows as we can afford it. Also considering some more foam in the hot roof. Any help or links is greatly appreciated!

When it is close, you need some experience with the program. Your heating contractor should provide a proper heat load that looks something like this the attached.

If you tighten your house up, you have to consider an energy recovery ventilator, more especially if you suffer long winters like we do here in Minnesota, similar to yours in AK. No one can, or should guess about heat loads e.g. "drafty" and "a lot of glass" don't really help the designer though both are critical to estimating heat loads. If we go with your numbers, discounting x.95 efficiency factor the 104mtuh output of the 110 would appear to cover it, just. Unless you have small zones (controlled by thermostats) that might short-cycle the boiler and considering you currently have a high temperature system with fin-tube baseboard and fan coil, you are likely to use the low fire without short-cycling.

When in future you add a slightly lower temperature radiant floor the minimum fire of the boiler may come into play. If a boiler is over-sized such as your current boiler, it will short cycle during the coldest weather burning more fuel and leaving you less comfortable. A modulating condensing boiler such as the Lochinvar Knight you are considering will modulate the flame after considering the outdoor temperature and only fire to the design water temperature your contractor programs into the machine.

So there are several things to consider before you size your new boiler. First is an accurate heat load for each zone. An over-sized boiler will short-cycle (go on and off) and suffer the same problem a city cab has with stop-and-go traffic, high maintenance and fuel costs. The Knight has a minimum fire rate and the higher the output, the higher the minimum fire-usually. The notable exception to this rule is the estimable IBC boiler made in Canada which had one of the lowest low-fire outputs and ranges to 150mbtuh inputs i.e. 150,000 British Thermal Units per hour. Think of a BTU as the heat generated by an old fashioned stick match.
http://www.badgerboilerservice.com/boilerchoice.html

The new ACCA design temperatures are somewhat different from their prior published numbers due to how they're counting the hours. What's now a 99% outside design temp is what used to be the 97.5th design temperature. In a house as foam-tight at yours using -9F for a design temp would still have reasonable margin- you won't get cold even if the output is sized EXACTLY to the calculated heat load at -9F.

Using the fuel use against base-65 heating degree-days as a sanity check, 400ccf/31 days is 400/(24 x 31)= 0.54 ccf/hr, or (102,000 x 0.54=) 55,080 input-BTU/hr at (65F-12F)= 53 heating degrees.

That's (55,080/53=) 1039 BTU/heating degree-hour.

At a design temp of -9F you have (65- -9F=) 74 heating degrees

A temp at which you'd be burning (1039 x 74=) 76,886 BTU/hr in source fuel.

At ~300K in and a DOE capacity of 225K out, you're looking at a ~75% efficiency system, but if it's been kept up and an automatic flue damper had been added it might run something like 78% AFUE. To be safe lets assume it's 80%.

Unless you used INSANELY deep overnight setbacks to achieve your January fuel use it's unlikely that your heat load is much more than that, and if the efficiency of the previous system is lower than 80%, the heat load is going to be LOWER than that.

The water temp required to deliver that heat with 165' of baseboard will be above condensing temp, but not outrageous. At 180F average water temp the baseboard could deliver ~100KBTU/hr into the house, but at a heat load of 69,000BTU/hr that's only ~420BTU/foot , which the baseboard could deliver at an average temp of ~155F. Assuming the pumping rate delivers a about a 20F delta-T that would mean the return water temp would be about 145F, a temp at which the boiler is only delivering ~87% efficiency.

The output of an 80K burner at 87% efficiency is (80,000 x .87=) 69,600BTU/hr or just above your -18F heat load number based on 80% efficiency.

Given that the actual efficiency on the beastie boiler is actually less than in the above analysis (and could even be less than the 75% nameplate ratings, rather than the 80% estimate used for margin) means your real 18F heat load is likely under 65KBTU/hr, and you actually have REAL margin even at -18F. And you would have PLENTY of margin using the ACCA's outside design temp of -9F.

The fact that you're not actually keeping the (very leaky) garage at 70F, but rather holding the line at 45 is a significant reduction in heat load, which accounts for the lions-share of the difference between ~100K as calculated by the program and the sanity check of ~69K based on fuel use/HDD. To be sure the heat load can't be higher than the source-fuel BTUs that kept the place at your desired temp, which is what it would have to be if the heat load was truly ~100K.

So, double-check the actual fuel use billing against average outdoor temps to be sure it's consistently in the same range over a few winter billing periods, but it seems very unlikely the 80K boiler would ever leave you cold. Mind you, the actual number of days between meter readings on the billing counts- there will even be some wiggle-error based on the time of day the meter got read if it was different, but even a 24 hour difference (30 vs 31 days) is only a ~3% error, which inserts only a ~2000 BTU/hr error on the heat load estimate.

Thanks for your replies. I was hoping that each of you would chime in! Thanks for setting me straight on the design temp, I just did a quick search and probably got and old chart. I, too think that the 80K unit would suffice. Especially after your calculations Dana from the gas usage, since you did not deduct for the hot water, drier and stove which would give me even more margin. And, there is also the gas fireplace insert for extra Btu's when needed. If I can follow your examples (not certain where some of the numbers came from, but still looking it over), I'll run the figures for a couple of other months. And Badgerboiler, I already have plans for an HRV as it got very moist and stale inside last winter. Not as easy to retrofit in an old house without any ducting, but I think I have a solution for the most important rooms.

Again, I thank each of you for your time and knowledge in replying. I'm working with my local dealer on a self install. They said that as long as I get permits and allow them to do the first fire my warranty will be intact. And I can get some tech help from them. I'm certain I will have some more questions for you guys as well. Hopefully we won't be without hot water for long or the wife will get unhappy!

The hot water, dryer & cooking fuel use is truly in the statistical noise of the heating use number on a winter month's billing period (especially if you use overnight setback on temperature) which is why I don't bother trying to factor it in. The actual vs. DOE nameplate efficiency would usually be as-larger or larger error, unless you're taking hour-long showers or filling & draining a monster-spa tub on a daily basis.

The number of days in the billing period is pretty important though, so be careful about that when using the existing boiler's fuel use & efficiency to estimate the whole heat load at design temp.

The nameplate efficiency of the old '70s boiler is steady-state output-BTUs/input-BTUs which 225/300 , or 75%. Post installation modifications such as electronic ignition vs. standing pilot and automatic flue dampers can increase the AFUE performance a few points but not the DOE efficiency, so using 80% in the analysis is giving it a pretty hefty margin. Most 40-50 year old boilers won't be hitting their DOE numbers unless the heat exchanger plates have been cleaned up, and even then the natural corrosion/erosion of the surface features on both the fire & water sides will have been reduced, decreasing the heat transfer efficiency. A good burner tech could measure the raw combustion efficiency if you cared about giving it more precision, but since the real number will never exceed the DOE efficiency, the error is in the direction that gives more margin on the real load, not less. So even if the analysis yields a number that's dead-on your 87%-efficiency output of the mod-con, in reality you will still have some margin, and it won't leave you cold.

Ok, picking up everything tomorrow, including the Lochinvar wall hung 85000btu and an indirect tank, along with a plethora of parts. I also received my stuff from Webstone valves. I was going to ask for opinions on those, but ran out of time. They are pretty expensive, but I think will save some time. I got the hydrocore manifold, the near boiler piping kit, and all the hardware to hook up the indirect (not the tubing). Pulled the permit with the city today and plan on starting the job when I go on days off on the 21st. I have been studying the manual and hope it will go smoothly, without too many days of no hot water. I'll actually have all the water off for a while, as I need to move the incoming water line from the well, move the pressure tank and re-plumb it all as well as move some of the wiring of the well controls. Also remove the shelf everything is up on (I think the tanks will be more stable sitting on concrete). I guess we'll be showering in the RV for a couple days.

My supplier told me that if I have the job properly permitted and then have a licensed plumber come and do the initial fire up and tests my warranty will be intact. I called a plumber who is a friend of a friend. He came out to look over the job. He said not to use the outdoor reset, as they just cause problems, and the near boiler plumbing loop is unnecessary. Guess he's old school! In my opinion, it is silly to buy a high efficiency boiler and not install it properly, therefor never reaching the high efficiency range. Hopefully I'm on the right track.

If you set the thing to a fixed output temp of 150F or higher you will never get more than ~87% efficiency out of it, whereas with outdoor reset you'd probably average in the low to mid-90s. I haven't looked at what it takes to set up and tweak ODR for this unit, but as an educated & capable homeowner paying the bills it's well worth reading the manual and dialing it in.

If you had enough radiation to meet the design-day load with 120F water the difference in efficiency of going fixed-temp vs. ODR would be less, but still measurable.

A "problem" for a contractor is different than a homeowner. As a heating you make some calculations or your best educated guesses as to where reset curve needs to be, spend an hour reading the manual and setting it up, then STILL get the callback from the client if the curve is off to the point that the radiation doesn't keep up with the load under some condition, or when there's a sensor/cable connector failure, etc.. A callback is a big problem for HIM, wasting profitable time driving out to basically do some minor-diddly that he can't bill for, maybe even multiple times until the customer is happy.

But if you know how to install, program, and debug the thing yourself, making the adjustments is not going to be a problem for YOU. Just be sure to place the outdoor sensor where it won't give false readings from being in direct sun or the output of your clothes dryer vent or be buried in snow from a roof avalanche something. I'm sure there are many creative ways to screw it up that I haven't even considered, but just note that not all outdoor placements are ideal and do the best you can.

Dana's sympathetic argument for leaving the ODR in the box, is typically erudite, but in no way excuses the unprofessional practice of leaving out what you don't have the acumen or initiative to understand. ODR is critical to comfort and economy and can severely increase operating cost of a condensing boiler in certain applications. (Not as much as the all-to-common practice of severely over-sizing the boiler).

I coach my clients in the benefits and proper use and care of their new high efficiency boilers and have instructed many (men and women) over the phone to optimize the boiler in the coldest weather. If the client is not interested or incapable, we build the cost of first season service into the installation.

If you are in Alaska or Montana, a callback can certainly hurt you, but ODR reset simply isn't that hard to cover.

Yep, he just wanted to set the temp at 180 and leave it at that. Not running the near boiler plumbing loop is also less than ideal, I think.

I pulled the old beastie out today. What a tank! Don't know what it weighs, but it was more than two of us wanted. We were able to tilt and slide it off the shelf without ever lifting the whole thing. I put it on craigslist-are they worth anything, or just scrap price?

Some observations: Plumbing has 4 zones with Taco zone valves and two grondfos pumps. There was what I think is a aquastat/zone controller plumbed into the output, but I don't think it was wired completely or properly. It had terminals marked thermostat, but the wires coming from them were wire nutted together. It had a wire to the gas control and fired the burner according to water temp, I'm sure. It had line voltage going to the pump, but near as I can tell it just ran 24/7??? Even with all zone valves closed? Reason I say that is I separated the wires from the thermostat terminal (that were nutted together) and it would start the pump when touching them together, even with all zones off. The thermostats were hooked directly to the zone valves. Am I missing something?

The zone valves and pump are on the return side, my new boiler only shows diagrams of them on the supply side. Does it matter which way they are hooked up to the near boiler plumbing loop on the new boiler? The boiler has ability to control 3 zones, but I'll have 5. I'm assuming I need some type of zone controller? The plumbing diagrams in the boiler manual all show 4 zones, but it says nothing about the boiler only controlling 3 or what to do about it when you have more.

The second pump is on the hot tub loop. It's powered and controlled by the hot tub controls, but the confusing part is that there is a taco zone control on that loop as well. Is it necessary to have a zone control valve as well as control over the circ pump? It would seem just the pump and a check valve would be simpler. I think the pumps have a check valve in them anyway?

I'm adding a loop for a garage rad/fan unit. It came with a 110v thermostat to control the fan. How do I interface that with the zone valve? Is that part of the zone controller also?

To control a fan coil zone that has a 110F aquastat is simply a matter of feeding it water of sufficient temperature- the zone-valve or pump controlled by the T-stat or zone-controller should take care of it. With fin-tube baseboard the output is a bit less predicatable at 110F anyway, so the bottom of your ODR curve is probably going to be something like 115-120F anyway, and even if you set it lower, the fan-coil zone is only calling for heat when it's well below 45F outside, where the water temp required for your other zones is likely to be above 120F.

With some mod-cons it's better to pump toward the boiler pressurizing it slightly, with others it's better to pump away, but the placement of the pump affects where the expansion tank lives- you always pump-away from the expansion tank.

If you're not sure where your going with the system design with the equipment you have, it's probably better to let a pro spell out the schematic design. Given that the local installer doesn't even want to deal with ODR, it's probably better to look elsewhere for hydronic system design help.

BTW: Running fixed temp @ 180F out you'd probably only average ~85-86% efficiency, and a right-sized cast-iron beastie would use about the same amount of fuel. When you pay the premium for a mod-con you'd really want both the efficiency and comfort advantage, neither of which would be evident running it fixed temp @ 180F. If you had enough radiation to be able to run it at ~130F or less on design day you'd still get the efficiency out of it though.

To control a fan coil zone that has a 110F aquastat is simply a matter of feeding it water of sufficient temperature- the zone-valve or pump controlled by the T-stat or zone-controller should take care of it. With fin-tube baseboard the output is a bit less predicatable at 110F anyway, so the bottom of your ODR curve is probably going to be something like 115-120F anyway, and even if you set it lower, the fan-coil zone is only calling for heat when it's well below 45F outside, where the water temp required for your other zones is likely to be above 120F.

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Thanks, Dana. I removed the rad/fan heater unit from a hangar. It had a 110v (line voltage) thermostat on the wall which simply turned the fan on and off. It was a multi unit hangar, and I'm assuming the water circulated all the time regardless of demand (there were 10 individual T-hangars in one building with one boiler). So my question is-if I use that line voltage thermostat to control the fan, how is it interconnected with the zone valve so that water is only supplied when the thermostat turns the fan on? Or is there a better way, such as a normal low voltage thermostat with a relay to control the line voltage for the fan. Thanks.

Many/most air handlers and fan coil units come already equipped with aquastats to switch power to the fan. Yours doesn't have one, but it can be retrofitted, replacing the line voltage T-stat. It's probably easiest to retrofit with adjustable strap-on aquastat directly the pipe feeding the heating water to the coil (be sure to use one that has sufficient 120V ratings to handle the load), then use a standard low-voltage thermostat to control that zone wired to your zone-controller/zone-valve setup.

When the zone valve to the fan coil opens, water flows, the temp on the feeder pipe rises to the setpoint of the aquastat causing it's contacts to close, turning on the fan. When the flow stops, the temp drops, and the aquastat switches off (with some built-in hysteresis on temp).

A common easy-retrofit strap-on aquastat that will probably work is the Honeywell L6006C1018. (It's rated for 8A @ 120V- hopefully that's enough.) Wire it to close-on-rise, and be sure to set the temp high enough that it won't come on by itself during the summer heat, but low enough that it's guaranteed to come on a the low end of the ODR curve. (100-110F will probably work just fine.) Since your setpoint is below 50F you'll still get effective heat transfer and a big delta-T out of the coil even with 100F water.

Well...I'm the worlds slowest plumber, I think. It's been nearly a week! I got all the zone supplies and returns done. Made a new return manifold with seven connections (using 5 zones now, may want to add a couple in the future). My torch was not hot enough to do 1 1/4, I discovered after spending all afternoon literally waiting for minutes to get a joint hot enough to take solder. Didn't trust my joints, so I tested with air pressure and had a half dozen leaks! Glad I did. With the new torch got them all fixed and now can solder 1 1/4 easily. I was able to re-use the supply manifold after modifications. When I tested it with air I discovered one of the zone valves wasn't shutting off-so changed that also. I bought one of those Webstone near boiler plumbing loop kits with the Hydro-core manifold. Expensive, but slick. Put in a new spiro-vent, didn't really have much for air separation before. Seems like the majority of the work is done....we'll see. Wish I could get photos to post. I keep getting a "can't write temporary file" error when trying to link them.

Still have to demo the rest of the old shelf. Didn't want to cut the hot water off till I had to (water heater was on shelf next to old boiler). So that will come out tomorrow. Also need to move the well pressure tank so the water will be completely off for a while. I still have to redo the gas piping and run the intake and vent. There was also no sheet rock on the wall under the shelf, so that has to be done as well as some line voltage wiring (moving a couple boxes)....never ending!

Thanks Dana for the info on the fan coil. I've got it hung and all plumbed in, I'll probably wire it last as it's not needed just yet.